(Shell)+ssh_agent integration

Use the same socket file for any zsh shell I create (default shell for
user and hence from tty) so ssh-agent works consistently.

Shell/.zshrc

@@ -1,6 +1,7 @@
 # zshrc -*- mode: sh; lexical-binding: t; -*-
 
 export DOTNET_ROOT=~/.local/src/dotnet
+export SSH_AUTH_SOCK="${XDG_RUNTIME_DIR}/ssh-agent.socket"
 ## Variables
 TERM=xterm-256color
 ## Aliases